1. Field of the Invention
The present invention relates to cold-formable steel strip that has a corrosion-resistant coating.
2. Description of Related Art
The present invention relates particularly but not exclusively to steel strip that has a corrosion-resistant metal coating and can be painted and thereafter cold formed (e.g. by roll forming) into an end-use product, such as roofing products.
The present invention relates particularly but not exclusively to a corrosion-resistant metal coating in the form of an aluminium/zinc alloy.
The present invention relates particularly but not exclusively to high tensile strength steel strip.
The term xe2x80x9chigh tensile strengthxe2x80x9d is understood herein to mean that the tensile strength is at least 350 MPa.
The present invention relates particularly but not exclusively to metal-coated steel strip that is produced by a hot-dip coating method.
In the hot-dip metal coating method, steel strip generally passes through one or more heat treatment furnaces and thereafter into and through a bath of molten coating metal held in a coating pot. The coating metal is usually maintained molten in the coating pot by the use of heating inductors. The strip usually exits the heat treatment furnaces via an elongated furnace exit chute or snout that dips into the bath. Within the bath the strip passes around one or more sink rolls and is taken upwardly out of the bath. After leaving the coating bath the strip passes through a coating thickness station, such as a gas knife or gas wiping station at which its coated surfaces are subjected to jets of wiping gas to control the thickness of the coating. The coated strip then passes through a cooling section and is subjected to forced cooling. The cooled strip thereafter passes successively through a skin pass rolling section (also known as a temper rolling section) and a levelling section. The main purpose of skin pass rolling the strip is to condition the strip surface (with minimal thickness reduction) to smooth the surface. An additional benefit of skin pass rolling is to flatten surface defects, such as pin-holes and surface dross, when such surface defects are present. The purpose of levelling the strip is to deform the strip so that it is sufficiently flat for subsequent processing, for example in a paint coating line operating at high speed (i.e. at least 100 m/min). The skin pass rolled and levelled strip is coiled at a coiling station.
A major market for metal-coated, particularly zinc/aluminium coated, steel strip is as a feedstock for paint lines that apply a paint coating to the surface of the steel strip. Paint line products have a range of commercial applications and in the majority of cases it is necessary to cold form (such as by roll forming) the painted strip in order to produce final end-use products, such as roofing products.
It is important that metal-coated steel strip that is produced by a metal coating line, such as a hot-dip metal coating line, for use ultimately as a cold forming feedstock be produced reliably with properties that confer adequate formability under the cold forming operation. More particularly, providing cold forming operators with coils of painted metal-coated steel strip feedstock that behave consistently and reliably during a cold forming operation is an important consideration for the operators. Specifically, consistent quality cold forming feedstock enables operators to produce cold-formed product of a consistently high quality without having to make significant adjustments to cold forming equipment to compensate for coil to coil variations in the cold forming properties of the strip.
Cold formability of painted metal-coated steel strip feedstock becomes increasingly important with higher tensile strength steel strip, which is inherently more difficult to cold form.
A general object of the present invention is to provide a method of producing cold-formable, metal-coated, steel strip consistently and reliably.
A more particular object of the present invention is to provide a method of producing metal-coated steel strip that has high quality surface finish and consistent and reliable cold formability compared to currently available steel strip.
In the context of the present invention, the criteria according to which cold formability is assessed include:
(i) quality of the roll-formed profilexe2x80x94considered in relation to parameters such as severity of imperfections, two of which are oil canning and edge ripple;
(ii) performance in a roll former; and
(iii) consistency of the form and shape of the roll formed profile.
According to the present invention there is provided a method of producing a metal-coated steel strip which includes the steps of:
(a) forming a metal coating on a steel strip; and
(b) conditioning the surface of the metal-coated steel strip by smoothing the surface of the strip, the conditioning step producing residual stress of no more than 100 MPa in the strip.
According to the present invention there is also provided a method of producing a metal-coated steel strip which includes the steps of:
(a) forming a metal coating on a steel strip;
(b) conditioning the surface of the metal-coated steel strip by smoothing the surface of the strip, the conditioning step producing residual stress of no more than 100 MPa in the strip; and
(c) forming a paint coating on the conditioned strip.
The present invention is based on the realisation that residual stress in metal-coated steel strip, particularly high tensile strength steel strip, causes problems during cold forming (such as roll forming) the strip.
In particular, the present invention is based on the realisation that the conventional practice of levelling metal-coated steel strip, particularly high tensile strength steel strip, that has been skin pass rolled can introduce considerable amounts of residual stress in the strip and thereby affect adversely the cold formability of the strip.
More particularly, the present invention is based on the realisation that rolling metal-coated steel strip, particularly high tensile steel strip, in order to condition the surface of the strip (by deforming the strip to produce a smooth surface) should be carried out under rolling conditions that produce minimal residual stress within the strip.
In the context of the present invention, xe2x80x9cminimal residual stressxe2x80x9d is understood to mean residual stress of no more than 100 MPa.
In addition, in the context of the present invention, xe2x80x9cresidual stressxe2x80x9d is understood to mean the residual stress through the thickness of the strip. Accordingly, references to xe2x80x9cresidual stressxe2x80x9d herein should be understood as references to through-thickness residual stress.
It is relevant to note that there are two distributions of residual stress in strip. One is the through-thickness distribution mentioned in the preceding paragraph and the other is the distribution of residual stress across the width of the strip. The across-width distribution of residual stress is usually of small magnitude in the case of thin strip.
Preferably, step (b) of conditioning steel strip produces residual stress of no more than 90 MPa through the thickness of the strip.
The applicant has found that producing metal-coated steel strip, particularly high tensile strength steel strip, with minimal residual stress makes it possible to consistently and reliably roll form the strip.
Preferably the steel strip is high tensile strength steel strip.
Preferably the tensile strength of the steel strip is at least 400 MPa.
More preferably the tensile strength of the steel strip is at least 450 MPa.
Preferably step (a) of forming the metal coating on the steel strip includes recovery annealing the strip before forming the metal coating on the strip.
Preferably step (a) of forming the metal coating on the steel strip includes hot-dip metal coating the strip in a bath of molten coating metal.
Preferably step (a) of forming the metal coating on the steel strip includes the steps of recovery annealing steel strip, thereby producing high tensile strength steel strip, and thereafter hot-dip metal coating the strip.
The term xe2x80x9crecovery-annealingxe2x80x9d is understood herein to mean heat treating steel strip so that the microstructure undergoes recovery with minimal, if any, recrystallisation, with such recrystallisation being confined to localised areas such as at the edges of the strip.
Preferably step (b) of conditioning the steel strip smoothes the surface of the steel strip so that it is suitable for painting in a paint line.
Preferably step (b) of conditioning the steel strip smoothes the surface of the steel strip so that it is sufficiently smooth for painting in a paint line operating at least at 80% of its rated maximum production line speed.
Preferably step (b) of conditioning steel strip maintains the strip sufficiently flat for painting in a paint line.
The term xe2x80x9csufficiently flatxe2x80x9d is understood herein in the context of complying with appropriate national standards, such as Class A and Class B flatness specified in Standard AS/NZ 1365.
Preferably step (b) of conditioning the steel strip includes rolling the strip.
The rolling conditions may be selected as required to condition the surface of the strip and to produce residual stress of no more than 100 MPa.
Preferably the rolling conditions are selected to produce residual stress of no more than 60 MPa.
More preferably the rolling conditions are selected to produce residual stress of no more than 50 MPa.
More preferably the rolling conditions are selected to produce residual stress of no more than 30 MPa.
Appropriate rolling control parameters include, by way of example, any one or more of:
(i) strip extension;
(ii) roll force;
(iii) roll bending; and
(iv) entry and exit tension.
Preferably the metal-coated steel strip has a thickness of no more than 1 mm.
More preferably the metal-coated steel strip has a thickness of no more than 0.6 mm.
According to the present invention there is also provided a metal-coated steel strip having a residual stress of no more than 100 MPa.
Preferably the steel strip is high tensile strength steel strip.
Preferably the tensile strength of the steel strip is at least 400 MPa.
More preferably the tensile strength of the steel strip is at least 450 MPa.
According to the present invention there is also provided a metal-coated steel strip that is suitable for use as a feedstock for a paint coating line and has a residual stress of no more than 100 MPa.
Preferably the steel strip is high tensile strength steel strip.
Preferably the tensile strength of the steel strip is at least 400 MPa.
More preferably the tensile strength of the steel strip is at least 450 MPa.
According to the present invention there is also provided a feedstock for a paint coating line produced by the above-described method.
Preferably the feedstock is high tensile strength steel strip.
Preferably the tensile strength of the steel strip is at least 400 MPa.
More preferably the tensile strength of the steel strip is at least 450 MPa.
According to the present invention there is also provided a painted, metal-coated, steel strip having a residual stress of no more than 100 MPa.
Preferably the steel strip is high tensile strength steel strip.
Preferably the tensile strength of the steel strip is at least 400 MPa.
More preferably the tensile strength of the steel strip is at least 450 MPa.